1. Field of the Invention
The present invention relates to a shift control system for an automatic transmission, specifically to a safe shift control.
2. Description of the Prior Art
Generally, an automatic transmission for an automotive vehicle is provided with a torque converter and a shift gear mechanism of which a plurality of frictional elements, such as a clutch, brake are selectively actuated to switch a power transmitting path to thereby establish a desired shift stage automatically among a plurality of shift stages of different speed ratios.
The automatic transmission is also provided with a hydraulic control circuit for supplying a hydraulic pressure to actuators of the frictional elements selectively to get the desired shift stages.
This kind of automatic transmission is provided with a R-range for reverse movement, a 2-range or S-range, and a 1-range or L-range for fixing or restricting the shift stage obtained therein in addition to a D-range for a forward movement. The automatic transmission is further provided with a N-range for providing a neutral condition, a P-range for parking and the like. A driver operates a shift lever to select a desired shift range within which a shift operation is automatically made for establishing a desired shift stage.
If the driver operates the shift lever to select the R-range by mistake when the vehicle is in a forward movement under for example, the D-range, frictional elements for accomplishing the rear movement in the R-range are engaged so that an engine brake is effected abruptly. As a result, the frictional elements are subjected to excessive load which deteriorates the durability thereof.
As shown in Japanese Patent public disclosure (JP A2) No. 63-13949, there has been proposed that when a shift for the R-range is detected in the case where the vehicle speed is greater than a predetermined value, a solenoid valve is actuated to drain a hydraulic fluid introduced into the frictional elements which is supposed to be engaged in the R-range so that the shift gear mechanism is forced to be neutral.
Generally, the reverse movement can be obtained by engaging two frictional elements (for example, a low reverse brake and reverse clutch). Therefore, in order to prevent the gear mechanism from establishing the reverse movement in the R-range, it is enough to prevent one of the frictional elements from being engaged. If the driver operates the shift lever to select a forward range again thereafter by noticing that he or she had mistakenly operated the shift lever, the other (for example, the reverse clutch) of the frictional elements for the R-range is released to enable to establish a forward range.
When the release of the other frictional element (reverse clutch) is unduly delayed as the driver operates the shift lever to the forward range again, the frictional element for the rear movement and the frictional element for the forward movement may be concurrently engaged temporarily to cause a double lock. As a result, the frictional element for the rear movement (reverse clutch) is dragged to deteriorate the durability.